1. Field of the Invention
The present invention relates to fixed codebooks of speech codecs, and more particularly, to a method and apparatus for implementing fixed codebooks in a code excited linear prediction (hereinafter, referred to as CELP) structure and an algebraic codebook technique.
2. Description of the Related Art
The term “codec” is a compound word made up of the word “coder” for converting an analogue signal into a digital signal and “decoder” for converting a digital signal into an original analogue signal. A speech codec serves to convert an analogue voice signal into a digital signal composed of a relatively small amount of data, and transmits the digital signal to a distant place. The speech codec serves to convert a received digital signal into an analogue voice signal recognizable by a human being. Most speech codecs developed so far use algebraic codebooks as fixed codebooks. The entire structure of the algebraic codebooks is the same as a code excited linear prediction (CELP) structure. A technique obtained by combining the algebraic codebook technique with the CELP structure is referred to as an algebraic code excited linear prediction (hereinafter, referred to as ACELP) technique. Current speech codecs are the ACELP.
FIG. 1 illustrates an entire structure of an ACELP technique. As shown in FIG. 1, a speech codec with a general code excited linear prediction (CELP) structure 120 is constructed with three modules. A fixed codebook module 121 generates an excitation signal and transmits the generated excitation signal to an adaptive codebook module 122. The adaptive codebook module 122 functions as a human vocal chord. The adaptive codebook module 122 adds a pitch component to the excitation signal and transmits excitation signal to a linear predictive coding (LPC) synthesis module 123. The LPC synthesis module 123 generates a final voice signal by mimicking the shape of a human mouth by using a tenth all-poll filter in the case of a narrow band signal or a sixteenth all-poll filter in the case of a wide band signal. The aforementioned speech codec structure is referred to as the CELP structure 120. The ACELP technique was developed by combining an algebraic codebook technique 110, which is one of various algorithms of a fixed codebook, with the CELP structure 120. Hereinafter, when the algebraic codebook is exemplified, the algebraic codebook may be used as a term indicating a fixed codebook.
However, although a single technique such as ACELP is used, various speech codecs are used in connection with various systems, standardization groups, and available fields. Accordingly, a user has to use a device in which a plurality of speech codecs are embedded or has to use a plurality of devices accessible to various systems so as to access the various systems. For example, when a user has to access various systems such as code division multiple access 2000 (CDMA2000), wideband code division multiple access (WCDMA), and voice-over-Internet protocol (VoIP) systems by using one communication terminal, various speech codecs currently used or likely to be used in the near future, such as enhanced variable rate codecs (EVRCs), 13k qualcomm code excited linear predictive coding (13k-QCELP), adaptive multi-rate (AMR), adaptive multi-rate wideband (AMR-WB), G.729, G.729.1, and the like have to be embedded in a chip included in the communication terminal. Accordingly, a voice processing chip has to have high performance so as to include the various speech codecs. This increases the size and cost of the chip.
FIG. 2 is a graph showing a calculation amount ratio of each module of a speech codec. In general, a module that performs the largest amount of calculation is an algebraic codebook module. FIG. 2 illustrates complexity of each module measured by an encoder of an AMR-WB that is a standard codec of the third generation partnership project (3GPP) and a standard speech codec of the telecommunication standardization sector of the international telecommunication union (ITU-T). In FIG. 2, an amount of calculation performed by an algebraic codebook module 201 is greater than 54% of the total calculation amount. Accordingly, it is necessary to decrease the complexity of the fixed codebook that performs the largest amount of calculation in the calculation of the speech codec to which the ACELP technique is applied, when the various speech codecs are embedded in a chip.